This invention relates to methods and apparatus for boring underground horizontal passageways.
Horizontally bored underground passageways for pipe lines and utilities such as electrical distribution lines provide a safe, economical and environmentally responsible alternative to digging through or building over the natural terrain and man-made obstacles.
Pending U.S. application Ser. No. 938,819, filed Sep. 1, 1992 discloses improved moling apparatus for forming a generally horizontal underground passage in soil for a utility conduit or the like that includes tool head structure with a base portion and a nose portion mounted on the base portion. The base portion is rotatable relative to the nose portion between a first position in which nose portion surfaces are symmetrical with respect to the tool axis so that the tool will move along a generally straight path and a second position in which nose portion surfaces are in asymmetric position with respect to the tool axis so that the tool will move along a generally curved path. That guided mole is preferably maintained in the straight line moling mole (axisymmetric tool shape) by continuously applying a slight torque to an elongated torsionally stiff air supply hose such that the rotatable nose portion is maintained against an internal stop which defines the symmetric configuration of the tool. To shift from the straight line (axisymmetric) configuration to a steered (asymmetrical) configuration, the body portion is rotated relative to the nose portion (which tends not to rotate relative the soil) by application of torque through the air supply hose to the base portion. Random vibration under moling action produced by the pneumatic impact structure could cause the rotatable nose portion to wander relative to the body portion and such action would tend to cause the tool to steer in a somewhat unintended and unpredictable direction.
In accordance with one aspect of the invention, there is provided moling apparatus for forming a generally horizontal underground passage in soil for a utility conduit or the like which includes tool head structure with a base portion and a nose portion mounted on the base portion. The base portion is rotatable relative to the nose portion between a first position in which nose portion surfaces are symmetrical with respect to the tool axis so that the tool will move along a straight path and a second position in which nose portion surfaces are in asymmetrical position with respect to the tool axis so that the tool will move along the curved path. The apparatus includes structure which interacts with the soil to impart a torque in a first rotational direction when the nose portion is in the first position and torque in a second (opposite) rotational direction when the nose portion is in the second (asymmetrical) position with respect to the tool axis.
In a particular embodiment, the nose portion is mounted on the base portion for rotation about a swash axis that is at an angle to the tool axis, and the nose portion includes rib structure in the form of flanges with leading edge portions that are inclined relative to the tool axis and tend through interaction with the soil to impart a torque in a first rotational direction about the nose portion axis. The flanges also have different projected areas when the nose portion is in the second (asymmetric) position, that differential flange area tending to produce rotation of the nose portion in the opposite direction. These torquing conditions tend to maintain the mole in a first condition (symmetric or straight ahead) when the nose portion is in the first position and impart a torque in the opposite direction which tends to maintain the nose portion in the second (asymmetric) position when the nose portion is in that position. Thus, greater stability of the respective moling conditions is maintained as a function of the position of the mole system.
In the first position, the nose portion applies torque that tends to make the entire mole spiral through the soil as it advances along a straight line path. In the second position the nose portion applies torque in the opposite direction. A slight torque applied to the air hose when in the second position counteracts the spiraling tendency and holds the tool in the desired steering direction. The mole is shifted from its straight to its steering configuration by a strong torque applied through the torsionally resistant air hose to the base portion. Since the nose piece is engaged with the soil, the mole body to which the air hose is rigidly attached, will then rotate relative to the nose piece and to the soil as the tool advances. That action shifts the nose piece from a straight configuration to a steered configuration against a stop and vice versa. The torque and the rotational motion applied to the tool via air supply hose must be sufficient to overcome the tendency of the nose piece to spin or rotate in the same direction during the shift. In other words, during shift, the tool body must rotate more quickly than the nose piece tends to rotate under propeller action alone. In a particular embodiment, it is possible to rotate the tool body almost in place, with little or no advancement of the tool during shift, by reducing the air supply to the tool during shift and/or momentarily running the mole in reverse during shift.
In another particular embodiment, two torque generating ramp surfaces in the form of grooves are provided on the base portion, one torque generating ramp surface being exposed in the symmetric nose portion position and generating torque in a first rotational direction as the mole moves through the soil and the other torque generating ramp surface being exposed in the asymmetric nose portion position and generating torque in a second (opposite) rotational direction as the mole moves through the soil.
other features and advantages of the invention will be seen as the following description of particular embodiments progresses, in conjunction with the drawings, in which: